Two cylindrical rods

Question:

You have two cylindrical rods of iron, identical in size and shape. One is a permanent magnet. The other is just non magnetized iron—attractable by magnets, but not a permanent magnet itself. Without any instruments, how can you determine which is which?

.

.

C

A

P

T

A

I

N

I

N

T

E

R

V

I

E

W

.

.

Solution: Take the two bars, and put them together like a T, so that one bisects the other (see Figure below). If they stick together, then bar B is the magnet.

Image.png

bar A

 

bar B

 

 

If they don’t, bar A is the magnet. That’s because bar magnets display no magnetic force at their centers since the two poles cancel out. So if bar A is the magnet, then bar B wouldn’t be attracted to its center. However, bar magnets are quite alive at their edges (i.e., the magnetic force is concentrated). So if bar B is the magnet, then it will attract the iron of bar A at any point.

Solution: Configure the two magnets like a T.

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